The present invention relates in general to the field of ultrasonic testing, and in particular to a new and useful apparatus and method for detecting flaws in coarse grained materials in which ultrasonic pulses are detected and stored in a digital memory and a computer or microprocessor is used to multiply the received pulses by a corrective phase shift factor to compensate for a time delay of the ultrasonic pulses from various angles of scan.
In the field of ultrasonic testing, it is known to scan a material to be tested either by using a plurality of transducers spaced around the material or by the use of a single transducer that is physically moved to various positions around the material to be examined. See for example U.S. Pat. No. 3,543,229 to Baum and U.S. Pat. No. 3,990,300 to Kossoff.
It is also known to convert the received ultrasonic signals from ultrasonic test equipment into digital form and to store them in a memory. See for example U.S. Pat. No. 4,163,393 to Gutierrez et al and U.S. Pat. No. 3,857,052 to Beller. The Gutierrez et al reference also teaches that comparisons between signals received from different locations of the transducer may be utilized to test for flaws. Gutierrez however cancels similar signals from the transducers. Beller on the other hand compares signals obtained at different time increments to detect a variation over time at the same point of the material to be tested. Neither reference teaches the multiplication of stored signals by a corrective phase shift factor to compensate for the time delay between the signals and to use the signals in an additive and averaging manner to provide reinforcement of the measured signals which increases a signal to noise ratio as in the present invention. These aspects of the invention will be set forth in greater detail later.
Conventional ultrasonic testing provides time, amplitude and spatial information that is combined through mechanical and electronic apparatus to form so called A, B, and C scans for flaw detection and sizing. The most commonly used technique is time-amplitude, A scan ultrasonic testing. All of these techniques make use of either a continuous wave or pulse excitation and time reference signal. The primary limitations with these techniques are that they only allow discrimination via signal amplitudes, time separation, or spatial separation which are variables extremely sensitive to the flaw orientation, test material properties, and test article geometry.
Typical ultrasonic testing makes use of a relatively narrow beam radiation pattern that is highly directional in material interrogation. When coarse grained materials (i.e., stainless steels, etc.) are encountered, the highly directional radiation patterns of the internal grain structures interact with the transducer pattern to provide high amplitude background signals. These signals are inseparable from those flaws yielding comparable amplitude levels when the conventional techniques are used. Under these conditions the probability of missing flaws and rejecting material due to "false flaws" is very high.
Previous inspection techniques used ultrasonic transducers that made a position-to-position interrogation of these materials where each position was an independent assessment of the material in the path of the beam. This technique has been accepted as adequate for fine grained material inspection but is less than desirable for coarse grained material. In these materials, absorption and scattering of the ultrasonic beam limits flaw detection capabilities to a greater extent than with other materials.